Syria's central city of Homs is under siege with armored vehicles moving through the streets and soldiers firing on residents, Associated Press reports.
Homs residents were hiding in homes and offices to escape gunfire, according to the Syrian activist network, The Local Co-ordination Committees.
Activists said at least 13 people were killed in the latest violence: eight in raids on the central province of Homs, four when a bus with oil workers was raked by gunfire, and a man shot dead trying to cross the border into Turkey, AFP reports.
The bloodshed came as International Committee of the Red Cross (ICRC) head Jakob Kellenberger ended a visit to Syria, where rights groups say 10,000 people have been arrested since anti-regime protests erupted in mid-March, AFP reports.
A senior Arab League official, meanwhile, said secretary-general Nabil al-Arabi will visit Damascus on Wednesday carrying an Arab peace initiative for Syria.
The Syrian Observatory for Human Rights said a man and his son were among those killed in Homs, adding they were apparently shot dead by "shabiha" pro-government militiamen as the two rode a scooter.
The Observatory also reported four people were killed and 11 wounded when gunmen at dusk ambushed a bus carrying employees of the Homs branch of Syria's oil company.
The Britain-based group said at least 35 people were wounded in Homs, where up to 80 people were arrested in the city's Al-Khalidiyeh district.
And a "mass grave containing the remains of seven bodies" was discovered near the village of Rami in northwest Syria, the LCC reported, adding that the army prevented residents from retrieving the bodies.
Meanwhile, U.N. Secretary-General Ban Ki-moon urged world leaders to take a strong stand against the Syrian government and its bloody crackdown on civilian protestors.
Ban has condemned the violence he says Syrian President Bashar Assad is perpetrating against innocent people during the five-month-old uprising against the government.
Mr Ban said it was time for U.N. members to unite and take "coherent measures", Associated Press reports.
A 2009 Wikileaks cable, by a top U.S. official in Damascus, has also been revealed which says Syrian diplomacy is "brutal" and officials are "not embarrassed to be caught in a lie", Al Jazeera reports.
Syrian diplomats use harsh verbal lashings to intimidate foreign diplomats, if they even bother to meet with them at all, the U.S. cable by diplomat Maura Connelly said.
It was written at a time when the Obama administration was considering restoring full diplomatic ties.
"The Syrian officials at every level lie... senior officials generally lie when they deem a topic too dangerous to discuss," the 2009 U.S.cable says, Al Jazeera says.
This comes as Syria has for the first time allowed Red Cross (ICRC) representatives to visit its main prison Tuesday and assess the condition and treatment of those detained by the Syrian authorities (read more at Global Post on Red Cross prison visit).